1.14.2 BLOX.BDropDownList: callbacks

callback: aReceiver message: aSymbol

Set up so that aReceiver is sent the aSymbol message (the name of
a selector with at most two arguemtnts) when the active item in
the receiver changegs. If the method accepts two arguments, the
receiver is passed as the first parameter. If the method accepts
one or two arguments, the selected index is passed as the last
parameter.